Automatic guidance systems are used to guide powered and unpowered aircraft toward desired targets. For example, the experimental airdrop system named “Snowflake,” jointly developed by the Naval Postgraduate School and the University of Alabama in Huntsville, is an autonomously guided unpowered parafoil (aerodynamic decelerator system or ADS). The system includes guidance, navigation and control (GNC) electronics that receive global positioning system (GPS) data and generate real-time flight control signals that control the parafoil via actuators. The GNC electronics attempt to cause the parafoil to fly a trajectory that matches a desired trajectory to a target on the ground. The system assumes the parafoil descends at a constant rate.
The desired trajectory includes flying directly toward the target until the system reaches a loitering altitude. The system then flies along an approximately circular (in top view) spiral path until the system reaches a lower altitude. While in the spiral path, the device estimates wind direction and wind speed by comparing GPS-provided ground speeds at various points along the circular path. When flying in the same direction as the wind, the ground speed is greatest, and when flying directly against the wind, the ground speed is least, thereby enabling the system to automatically determine when the parafoil is flying with the wind and when it is flying against the wind. From these measurements, the system estimates the wind direction. A difference between the greatest and the least ground speeds provides an estimate of the wind speed.
After leaving the spiral, the system flies in the direction of the wind, past the target, and then it executes a 180° turn to fly directly into the wind, toward the target. The location of the beginning of the 180° turn is calculated to cause the system to end the turn directly downwind of the target and at an altitude that will cause the system to land at the target, given the system's design air speed and design descent rate and the estimated wind speed. Landing directly into the wind minimizes ground speed at the landing, thereby providing a relatively soft landing.
Wind is, however, variable and unpredictable, leading to landing location errors. Improved GNC would provide more accurate landings.